Microstructure and electrical properties of lanthanum-modified Pb(Zr,Sn,Ti)O3(PZST) antiferroelectric (AFE) ceramics have been investigated by weak field dielectric, pyroelectric response, strong field polarization, and X-ray diffraction (XRD) techniques at different temperatures. Diffused phase transition and dielectric frequency dispersion (0.1—100kHz) have been observed within the temperature range of -100—50℃. However, XRD indicates that AFE ceramics are of tetragonal phase within the above-mentioned temperature range. Under a strong electric field, AFEt will be switched to a metastable ferroelectric (MFE) state, accompanying with the disappearance of dielectric frequency dispersion. Then the MFE state changes to AFE state again as temperature increases, producing a strong pyroelectric current peak. A new model considering compositional fluctuation on a nanometer scale in PZST antiferroelectric ceramics has been suggested to explain the polar relaxation in AFE state.